Extracting apparatus



Nov. 13, 1945. K. FINSTERBUSCH EXTRACTING APPARATUS Fil ed March 29, 1940 2 Sheets-Sheet 2 gaseous hydrocarbons or mixtures oi normally Patented Nov. 13, B45

UNITED; STATES PATENT oFrlcE,

area-m J This invention relates to improvements 'in 7 methods and apparatus for refining wide boiling range fluid mixtures, more particularly socalled unstabilized or excessively volatile liquids,

, and for separating or extracting desired constituents or constituent iractions therefrom.

Such mixtures are products of many and varied industrial operations such as. for example, the

. high pressure cracking of petroleum. Thus, there is commonly obtained, as the overhead product ll of the main iractionating tower of a high pressure cracking unit, a wide boiling range fluid mixture of gases and vapors including so-called fixed gases, e. g. H2, C01, C02 and heavier normally liquid and normally gaseous hydrocarbons. The condensate obtained generally by simple cooling of this overhead product rorms the socalled "pressure distillate, an excessively volatile liquid mixture from which, by suitable operagage, would separate item the combined liquid a substantially 91-0: free liquid as abottoms product. However, in order to obtain reflux without the use of expensive refrigeration, when v the generally available coolingwater is at temperatures of from about 70 F. to 100 F., the gaseous overhead product? from the de-ethanizer will normally contain from 15% to 30% or C:

hydrocarbons, in addition to the undesiredlighter constituents, depending upon the relative amounts or Cl and C: hydrocarbons contained in the charge. If high Ca recovery is desired as an adjunct to stabilization of the charge or as a primary consideration, it would be necessary to II incur additional expense and recycle this over-.

head product to a vapor recovery system fcrextraction of the C: constituents as was done in the case 0! the separator we gas. 7 The initial cost or such a de-ethanizer unit is relatively high tions, there may be extracted desired constituents I! ecause of the hi h pressure (550-650 lbs. per

and constituent fractions such-aa'ior example, light and heavy pressuredistillate, stabilized as to vapor pressure specification; propane, suitable for use. as "bottled gas"; and. other. normally gaseous hydrocarbpnutilizable for various 'pur-' p es.

By a common method of extracting a stabilized pressure distillate from such a mixture concomitant with the recovery of a high percentageo! Cs hydrocarbons, the mixture constituting the charge is cooled and thereby liquefied in part and passed to a gas-liquid separator from which the uncondensed portion, i. e. the "wet". we containing some of the desired Cs and heavier hydro- I carbons as well as lighter hydrocarbons and fixed gases, passes to a vapor recovery system by means of which the desired C: and heavier hydrocarbons are separated from the lighter onstituents and are recovered as a product in a li uid state. This is combined with the unstable liquid pressure dlstillate oi the separator for further processing. The combined liquid, however, will nevertheless still contain'some excessively volatile hydrocarbons .such as C: and lighter hydrocarbons unavoidably liquefied in steps 01' condensation, absorption, etc. but nevertheless undesired in the 4 final product because of their extreme volatility.

Stabilization or such a distillate is a costly procedure and these undesired lighter hydrocarbons in the combined liquid may be, and ordinarily are, eliminated bystraight fractionation in a so-called de-ethanizer. operating at the normal pressure of, for example,

from about 550 to about 650 lbs. per square inch companylng drawings, and the appended claim.

Such a de-ethanizer square inch) at which the unit must be operat'ed. The unit is also expensive to operate under these conditions because of the high reflux ratios required and the magnitude or the necessary cooling and heating utilities.

An object of my invention, therefore, is to provide a method for separating or extracting desired constituents or constituent fractions from wide boiling range fluid mixtures which will be more economical from the standpoint of operatingcost.

Another object of my invention is to provide apparatus for separating or extracting desired 40 from a liquid mixture of normally gaseous and normally liquid constituents containing the same without loss, or with a negligible loss, or desired less volatile constituents from the mixture to the end that a high percentage of the less volatile constituents may be recovered in a final product.

Another object .of my inventionis to provide an improved method and apparatus for refining pressure distillate or similar relatively wide boiling range derivative fractions for the purpose of In general, ,the advantageous results obtainable through the use of my invention are predicated upon a recognition of the fact; firstly, that at a given pressure and temperature there is a pronounced spread between the respective vapor-liquid equilibrium constants of the respective light and heavy constituents of a volatile fluid mixture; and secondly, that at a given pressure the ratio to each other of the respective vapor-liquid equilibrium constants of any two constituents increases with decreasing temperature. Thus, for example, for a mixture of normally gaseous and normally liquid hydrocarbons at a given pressure and temperature, the respective vapor-liquid equilibrium constants for the light hydrocarbons. methane and ethane, are higher than those for the heavier hydrocarbons, propane, butane, etc., and the ratios of the respective-equilibrium constants of, for example, methane to propane, or ethane to propane, increase as the temperature decreases.

In the accompanying drawings which form part of the instant specification and are to be read in conjunction therewith and in which like numbers refer to like parts throughout the several views:

Fig. 1 is illustrative of one form of apparatus suitable for carrying out the method of my invention.

Fig. 2 is illustrative of another form of apparatus suitable for carryin out the method of my invention.

In carrying out my invention, the fluid mixture from which the desired constituents or constituent fractions are to be extracted is subjected while in a liquid state and starting from a predetermined preferential vaporizing temperature, to a series of venting or flashing operations designed to secure preferential vaporization of certain undesired light constituents or constituent fractions without undue vaporization of the heavier constituents or constituent fractions. Each flashing operation of the series is conducted at a lower pressure than that at which the preceding operation was conducted, each pressure being carefully selected to eflect the desired preferential vaporization of undesired constituents or constituent fractions of the mixture at each operation and the-attainment of successively lower temperatures of the liquid due to cooling resulting from the latent heat of vaporization. The pressure reduction selected for each venting operation is preferably so chosen that the resulting temperature drop will ensure the maintenance throughout the series of a desired high ratio of the respective equilibrium constants of the undesired to the desired constituents.

The vapors and unvaporized liquid are separated from each other after each venting operation, the liquid being subjected to a succeeding venting operation conducted at a lower pressure. The vented vapors from each operation are preferably subjected to a vapor recovery operation for extraction therefrom and recovery in a liquid state of desired fractions or constituents which may be combined with or recycled to the mixture being processed at a suitable phase of the operation. In some cases it is desirable to discard vented vapors from one or more vents or the series when the content of desired fractions is negligible.

I have found that it is advantageous to carry out the flashing or venting operation by a series of steps from the initial to the final pressure rather than by a single operation from the initial to the final pressure. By this expedient, a greater amount of the undesired light constituents or fractions and a lesser amount of the desired heavier constituents or fractions are vented from the charge thus appreciably reducin the quantity of desired constituents or fractions to be recovered by the vapor recovery system and measurably reducing the cost of the overall operation.

Referring now more particularly to the accompanying drawings, I have shown in Fig. 1 a system for demethanizing by the use of my invention the overhead product obtained from the main fractionator of a high pressure liquid phase cracking unit concomitant with a high propane recovery in the stabilized distillate product. The overhead product, which product for convenience I shall term the charge, enters the system in a heated gaseous-vaporous state under substantial pressure through the pipe ID by which it is conducted, after temperature reduction and partial condensation in the coolers II and I2, into a gasliquid separator l3.

The uncondensed wet" gas portion is conducted from the separator l3 to a vapor recovery unit I4 by a pipe IS, the outlet pressure on the separator It bein automatically controlled and maintained at a predetermined value by the setting of the pressure controlled valve iii.

The vapor-recovery unit It (diagrammatically shown) may include the usual compressors, absorbers, coolers, etc., co-operative in a known manner to extract or effect a separation from the wet gas of those desired constituents which were unavoidably commingled with the undesired constituents of the wet gas portion discharged from the separator it. Thus, in the demethanization of the charge in the instant operation wherein one of the objects in view is the recovery of as high a proportion as possible of, for example, the Ca and heavier hydrocarbons in the charge, the vapor recovery unit I 4 will be designed to effect a separation of these constituents from the wet gas supplied to it through the pipe l5. These constituents are thereupon conducted in a liquid state from the recovery unit by the pipe I! into the feed pipe l8 wherein they are commingled with unstab'ilized pressure distillate flowing therethrough under the pressure in the separator II.

This combined liquid is conducted by the pipe ll into the first section IQ of the three-stage demethanizer shown. Cooling of the combined liquid to a preferential vaporizing temperature is eflected in its transit through the pipe It by means of water cooler 20 and exchanger cooler 2| while the rate of flow is automatically controlled by the setting of a valve 22 actuated in a known manner by a liquid level controlled device 23 responsive to variations in the liquid level in the separator l3.

The'cooled combined liquid entering the first flash section I! of the demethanizer vaporizes in part, due to the lower pressure, with a resultant attendant beneficial temperature reduction of the unvaporized liquid. The vapors and gases evolved by preferential vaporization in the flash section i8, consisting mainly of highly volatile C1 and C: hydrocarbons together with a relatively pressure than the wet gas.

The'pr'essure within the flash section II is maintained at a .predetermined value by the setting of a pressure controlled galve II and is such that in this, as well as in the succeeding flashing 'or venting steps. the vapor-liquid and heavier hydrocarbons will be below their boiling points.

Effective distribution of'the unvaporized downflowing liquid over a. considerable area in the flash section It may be assisted by means of a distributor 21 of the cone type delivering the liquid over the hut of a number of distributing trays or plates 28 such as are well known inthe art. Unvaporized liquid that collects in the lower part of flash section It flows through the pipe through the pipe 25 practically all of the equilibrium constant for Ca hydrocarbons preferably is less than one. Under these conditions,-

29, in quantities regulated by the setting of a valve til actuated in a known manner by the liquid level control device it in response to the liquid level in flash section it, into the second flash section 32 of the flash tower wherein due to the lower pressure therein maintained fur-v ther vaporization of the liquid and consequent section 82, like flash section is, is similarly equipped with'adistributor t3 and distributing trays it and is connected by vapor discharge pipe 35 with the header 25, the pressure within the flash section being automatically maintained by the setting of'the pressure controlled valve 38.

Unvaporized liquid that collects in the lower part of the flash section .32 is thereafter subjected to a third but not necessarily final stage of pressure and temperature reduction, beingand 32, is similarly provided with a distributor cone ill and distributing plates ll, and is connected by a gas vapor outlet pipe I! with the header 25. Pressure controlled valve 43 in the pipe-42. may be set to the desired pressure in the flash section 31.

It will be apparent that the successive pressure-reductionsin the flash sections I9, 32 and 31 are to be chos'en'with a view to establishing suchtemperature levels as will permit achieving .the desired beneficial preferential vaporization and elimination of C: and lighter hydrocarbons and retention of Ca and heavier hydrocarbons.

Ihave found a series of venting or flashing operations to be-preferable to a single venting or flashing operation from the initial to the flnal pressure since a greater amount ofthe undesired lighter fractions or constituents and a lesser amount of the desired heavier fractions l or constituentsis vented. Thus, in the instant demethanizing operation, the total of C: hydroq assays:

vapor every system since they are at a lower carbons vented by my step flashing system is less than would be vented in a sing-ls flash system and Y the total of C1 and Cs hydrocarbons is greater. By my step system. I am able to eliminate methane and practically 50% of the C: hydrocarbons present in the combined liquid feed to the demethanizer, with a venting of about 15% of the Ca hydrocarbons, about 5% I of the. pi hydrocarbons and practically none of the heavier Y hydrocarbons. Thus, the total vent gases flow-' ing to the/gas recovery system through the pipe will normally .contain,(o n a molal basis) less than of the Ca hydrocarbons and constitui5 temperature reduction is effected. Theflash outs heavier than Cehydrocarbons, and more than 70% 9! C2 hydrocarbons and constituents lighter than C: hydrocarbons. The successive evaporations produce .a temperature drop in the liquid, the magnitude of which depends upon. the concentration of the heavier constituents such as Cs and heavier hydrocarbons in the feed to the demethanizer. Ac-

cordingly, inmost cases, it will be found that the final liquid at g. in the instant operation, that in the third section 3'1 of the demethanizer, will be" from about 10 F. to about 1''. below the temperature of the combinedliquid feed leaving themally available as a cooling medium in the chargecooler ii and in the cooler 20. A. low temperature of the combined liquid entering the first secj cooler 2i and charge sub-cooler l2 beforebeing discharged from the system. The flow of this liquid through the pipe 44 is controlled by a suitable pump such as the centrifugal booster pump 48 at a rate regulated by a flow control valve Miactuated in a known manner by a liquid level control device 4'! responsive tovariations in the liquid level in the flash section 31.

. Residual gas such as. the hydrogen, Cl and Cs hydrocarbons uncondensed in the vapor recovery unit is released therefrom for other uses through the pipe W. The final demethanized liquid discharged irom the system through the pipe M may be fed to a stabilizer for production of stabilized pressure distillate of desired vapor pressure. In

such case, the stabilizer overhead product consistlng mainly of C2, C3 and Cs constituents may be condensed, or substantially completelyoondensed, at pressures below 350 lbs. per square inch gage .with cooling water at normal temperatures since this stabilizer overhead product will contain a negligible amount of methane. This overhead product condensate may in turn be fractionated to obtain fractions such as ethane, propane and butane for use as bottled gas" prodvucts or to obtain fractions suitable ascharging stock for the manuiactureof other products by alkylatlon or other operations.

- If de-ethanization of the final liquid in the pipe M is desired, this liquid may be ied to a deethanizer unit such as heretofore discussed. Because of the initial de-methanization, operation of the de-ethanizer may be carried out with a 7 lesser quantity of reflux and "with less loss of me. 2. it

will be observed that each flashing operation is combined with an absorption operation for concomitantly extracting from the vaporized constituents certain unavoidably vaporized constituents whose restoration to the unvaporized liquid for recovery as a final product, or-as part of a final product, is desired. Thus, in demethanizing a fluid mixture, such as, for example, unstabilized pressure distillate, wherein high propane recovery is one of the objects in view, the body of vaporized constituents resulting from each flashing or venting operation will perforce contain some proportion of the constituents such as Ca and heavier hydrocarbons whose recovery is desired. Restoration of substantially all of these desired constituents to the liquid being processed and their ultimate recovery may be economically and advantageously effected by absorption operations respectivel coincident with each flashing operation. a

As is depicted in Fig. 2, the wide boiling range charge in gaseous--vaporous state enters the system at an elevated pressure through a pipe 50 by which it is conducted, after temperature reduction in the water cooler i and exchanger cooler 52 productive of substantial condensation, to the gas-liquid separator 53. The uncondensed wet gas portion is conducted by a pipe 54 from the separator 53 into the lower part of an absorber 55 while the condensate flows from the separator through a pipe 56 at a rate controlled by a valve 51 actuated in a known manner by a liquid level controlled device 58 responsive to the liquid level in the separator 53'.

The wet gas rises upwardly in the absorber 55 through the elements of a series of tray banks 58, 50 and 5!, each of which comprises fractionating plates or trays such as are well known to those skilled in the art. The passage of the gas upwardly through the absorber is countercurrent to the passage of absorption oil downwardly therein. The absorption oil, preferably having substantially the same boiling range characteristics as the heavy traction oi the charge, enters the absorber in its cool lean state through a pipe 62 and, by absorption, extracts irom.the wet gas, in its downward progress through the tray banks, mainly those constituents thereof, such as Ca and heavier hydrocarbons, whose recovery is most desired. Intercoolers 63 and 84 are interposed between the tray banks 59-60 and 606l respectively, for removing the heat of absorption from the absorption oil as it passes downwardly through the absorber between the tray banks. The cooling medium for the intercoolers may be water, although if the temperature oi the normally available water is too high, a colder medium such as the sub-cooled liquid from the final vent stage may be used.

The unabsorbed dry gas which issues from the absorber through the header pipe 65 is conducted through a pressure controlled valve 65 from the system. The rich oil containing the absorbed constituents which, in the instant case, under the pressure and temperature conditions maintained in the absorber, will consist mainly of Ca and heavier hydrocarbons, flows .into the pipe 55 through a pipe 61 regulated by a valve 88 actuated in a known manner by a liquid level controlled device 59 in response to variations in the liquid level in the absorber 55.

The combined liquid in the pipe 58 is conducted thereby through the water cooler and exchanger cooler II by which its temperature is reduced to a predetermined value, into the first ot a series 0t deniethanizing flash-absorption towers l2, l3 and These towers are structurally similar. Each comprises a circulating head or channel 15 from which is suspended in each case a tube bundle 16, through which a suitable cooling medium supplied to the channel from a header pipe 11 is caused to circulate and is discharged through an outlet header 18. Thus, the respective channels are supplied with cooling liquid from the header pipe 1'! by the pipes 19, and 8| respectively. The liquid after circulating through the respective tube bundles is conducted from the respective channels to the header pipe 18 by the outlet pipes 82, 83 and 84 respectively.

Baiile plates 85 form dams upon which absorption oil collects in pools and flows downwardly around the tubes through annular holes formed between the respective tubes of each bundle and the baflle plates. These baiiie plates extend transversely of each of the towers l2, l3 and 14 and of the respective tube bundles at suitably spaced intervals and direct the flow of the rising vapors back and forth across the tubes which are wetted by the absorption oil in its downward flow thereover. The heat of absorption is removed from the absorption oil by indirect heat exchange with the cooling medium as the absorption oil flows in its downward path over the outside of the tubes of the tube bundle in each oi! the towers.

Absorption oil is supplied in its lean state from a header pipe 82 onto a distribution bafile 85' in each tower by means of the lean oil inlet pipes 86, 81 and 88 respectively and flows downwardly over each tube surface to the oil pool on the baiile plate beneath, annular spaces being provided between each tube and its baille plate for accomplishing this flow. Gaseous and vaporous constituents which are not absorbed'by the lean oil are conducted from the respective towers through the pipes 85, 80 and SI respectively into the dry gas header pipe 55 and are conducted thereby from the system. Pressure controlled valves 92, 93 and 84 situated in the outlet pipes 89, 90 and 9| respectively maintain the operating pressure 01' the respective towers 12, 13 and 14 at the desired valve.

Tray banks 84 respectively comprising baille or fractionating plates well known in the art extend transversely of the respective towers below the charge inlet opening and function to knock back" or de-entrain liquid particles from volatilized gaseous-vaporous constituents and to disengage the gaseous-vaporous constituents from the descending liquid.

The cooled combined liquid enters the first stage flash-absorption tower 12 through the pipe 55.

The reduced pressure maintained in this tower causes vaporization of a portion 01' the liquid. The undesired methane and some heavier vaporized constituents rise upwardly in the tortuous path i'ormed by the baflle plates 85 generally countercurrent to downflowing cool lean absorption oil. The heat of absorption is dissipated and the absorption oil maintained at a desired operating temperature by indirect heat exchange with the cooling medium circulating through the tube bundle I8. The cooling medium may be water, although I prefer to utilize for this purpose the subcooled final liquid obtained as a product of the last stage of the flashing operation. The equilibrium conditions maintained within the tower 12 are preferably such that a relatively minor quantity of Cl and C2 hydrocarbons vaporized will be absorbed by the adsorption oil and they will pass aeeaveo hum the tower through the pipe 89 into the dry gas header or residue gas main 6B. In the instant operation the equilibrium conditions established in the tower 12 through control of the pressure are preferably such that the vapor-liquid equilibrium nomic advantage.

It is also to be observed that because of the reduced temperature or the volatilized constituents consequent upon the flashing operation and because of the removal oi the heat of absorption, the quantity of absorption medium required for recovery oi the Ca and heavier constituents is appreciably reduced.

The rich oil from the upper or absorption section of the tower combines with the separated unvaporized liquid in the lower or flash section of the tower and is conducted to the second stage flash tower is by a pipe 95, the rate of flow of the liquid being controlled by ated in a known manner by a liquid level controlled device 01 responsive to variations in. the liquid level in the tower 12.

Flashing oi the combined liquid to eliminate more or the methane is again carried out, but at a predetermined lower pressure and consequent lower temperature, in the second stage tower i3, absorption the vaporized Ca and heavier constituents being repeated therein and the unabsorbed C1 and Cehydrocarbons being conducted from the tower through the pipe 50 into the dry gas header pipe 00. The combined liquid now at a lower temperature is conducted from the tower 13 to the third stage flash tower lt'throuizh a pipe 91, the rate of flow of the liquid being controlled by a valve 00 actuated by a li uid level controlled device 00 in response to variations in the liquid level in the tower 13.

Final flashing of the combined liquid for elimination oi the balance of the methane is carried out at a still lower predetermined pressure and consequent lower temperature in the third stage tower II, the unabsorbed C1 and C2 hydrocarbons being conducted from the tower through the pipe 9i into the dry gas header pipe 00.

The pressure at which the flashing is carried out in each of the flash towers 12, I3 and 14 is preferably so chosen that at the resultant equilibrium temperature preferential vaporization of the undesired constituents will occur. Thus, in the instant operation where demethanization is chiefly sought for the vapor-liquid equilibrium constant established for the Ca and for the heav ier h drocarbons by the pressure-temperature conditions shall be preferably less than one. Absorption recovery or such C: and heavier hydrocarbons as are vaporized under these conditions is more easily efl'ected and preferential vaporization oi the lighter undesired hydrocarbone is achieved since their equilibrium con= stants will be greater than one.

The final liquid which accumulates in the lower portion of the third flash tower H is withdrawn therefrom in its sub-cooled state by a centrifugal pump I00 through a pipe IM and is delivered into the header pipe ii at a rate controlled by the valve I02 actuated in a known manner by a liquid level controlled device I03 responsive to a valve 90 actu-- variations in the liquid level or the liquid in the third stage tower iii.

The sub-cooled final liquid, i. e. demethanized but as yet commercially unstabilized pressure distillate and rich oil supplied to the header pipe ii is admirably suited for cooling the absorption oil in the respective demethanizer flash towers by circulation through the respective tube bundles it, as has previously been described, A portion of this relatively cold liquid upon being discharged into the outlet header pipe l8 after coursing the channels 15 and tube bundles l6 may be utilized further, if desired, to cool absorption oil in the .intercoolers b0 and M. In such case it may be supplied to the intercoolers through the pipe H04 in suitable quantities regulated by the valve I05.

The remainder of the sub-cooled final liquid, or all of the liquid if none be used as a cooling medium in the intercoolers b3 and be, is conducted by a pipe I106 through the sub-cooler ii to a diagramatically depicted stabilizing unit ibi. The temperature of the combined liquid flowing through the sub-cooler ii to the first stage flash d methanizer tower it is reduced to a more fair rable value by indirect heat exchange with the sub-cooledliquid. A portion of the sub-cooled final liquid may be circulated by the pipe ltd through the exchanger sub-cooler bite-lower the temperature of the entering charge. Suitable valves i09 and lid in the pipe Mid and valve iii in the pipe l06 are operable in an obvious manner to divert the final liquid through the pipe it! in such quantities as may be deemed necessary. Such portion of sub-cooled final liquid as is caused to circulate through the inter-coolers 63 and M may be re-combined with the main stream coursing to the distillation unit i0l through the pipe I08. by means of the pipe I I2,

The final liquid may be operated on in the distillation unit ill! by suitable well-known methods of distillation, fractionation, rectification, etc., to separate the final liquid into the constituents or constituent fractions desired as products. For example, light and heavy pressure distillate fractions, each stabilized as to vapor pressure specifications, may be recovered in the unit and discharged as products through the pipes Ill and H5 respectively; and, a light liquid hydrocarbon fraction comprising mainly C3 hydrocarbons, together with such C4 hydrocarbons as constitute an excess over the quantity of C4 hydrocarbons required for vapor pressure speciflcations in the light pressure distillate fraction, and such C2 hydrocarbons as were not vented with the C1 hydrocarbons in the demethanizing operation, is discharged from the distillation unit as a product through the pipe NB. This light hydrocarbon i'raction may be further fractionated, if desired, to obtain fractions such as ethane, propane and butane for use as "bottled gas products or to obtain fractions suitable as charging stock for the manufacture of other products by alkylation or other operations.

The stabilized heavy pressure distillate fraction which corresponds generally to the heavy portion 01' the unstabilized pressure distillate in the separator 53 is suitable for use as an absorption medium in the absorber 6i and in the demethanizer flash-absorption towers i2, '13 and M. Accordingly, a portion thereof is conducted from the pipe M5 by the pipe ill into the lean oil headerpipe 62 for distribution thereby to the various absorber units as has been described above, the balance of the stabilized heavy pressure distillate being discharged from the system through the pipe i I 8.

Obviously, however, a separate absorption medium may be used if desired. In some instances, it may be undesirable or not feasible to combine the separate absorption medium in its enriched state with liquid such as that in the separator 53 which would normally be charged to the tower I2 in Fig. 2. In such case, the rich oil from absorber 60 may be processed as in Fig. 2 while the liquid in separator 53 may be flashed in a parallel series of venting operations carried out at successively reduced pressures in a multi-stage vent tower such as that of Fig. l, the vented vapors from the respective stages being conducted to the respective correspondingly pressured flash absorption towers of Fig. 2 wherein absorption recovery of desired constituents thereof may be carried out under optimum conditions concomitant with series venting and absorption recovery of the rich oil from absorber 00. Again, the liquid charge to the respective flash absorption stages may be supplemented by gases, including recycle gases, from sources outside of the system. Such gases could be introduced to the various absorption stages at suitable phases of the operation for recovery of desired constituents.

In each of the systems described in Figs. 1 and 2, each of the series of venting operations is made up of three stages. Obviously, however, a larger or smaller number of stages may be found desirable. In the actual application of my invention to the recovery from a relatively wide boiling range mixture of normally gaseous and normally liquid hydrocarbons, of a demethanized liquid product containing a high percentage of the C: constituents of the mixture, I have found it desirable to use as many as five stages.

In that recovery operation, the mixture in its gaseous-vaporous state was condensed as much as possible at the full operating pressure of 500 lbs. per sq. inch gage, by water cooling. Further cooling and condensation was accomplished by sub-cooling (after the water cooling) with a colder medium. At the high condensing pressure and relatively low temperature, approximately 85% of the Ca constituents and 95% of the C4 constituents present in the mixture were condensed. The desirably high percentage of these constituents condensed is attributable in part, at least, to the molal condensing effect of the C5 constituents and heavier constituents in the mixture. However, for the same reason, relatively large amounts of undesired methane and C2 constituents were also condensed, the quantity of methane condensed being approximately 30% of the quantity of C3 constituents condensed, and the quantity of C2 constituents being approximately 50% of the quantity of Ca constituents condensed. The total of C1 and C: constituents in such case represents about 80% of the total C3 constituents, or considering the total of C: and C4 constituents as a desired fraction, the total of C1 and C2 constituents formed about 4 of the C3-C4 fraction of the condensate, all percentages being on a molal basis.

In order to eliminate economically as much of the methane as possible, without undue loss of Ca and C4 constituents, the total liquid condensedwas subjected after further cooling to a 5-stage demethanizing operation in a system such as that of Fig. 1. In the first vent stage IS the pressure on the feed was reduced upon entry from about 500 lbs. per sq. inch gage to approximately 285 lbs. per sq. inch gage with an. ac-

companying temperature reduction. Under these circumstances, the relatively small quantity of vapor evolved contained approximately or methane and C: constituents and app xim y 20% of Ca and C4 constituents by composition. Actually the first venting operation eliminated approximately 30% of the methane from the feed and only 3% of the C: constituents so that the separation was very favorable. As an accompaniment to this venting operation, the total liquid was chilled by the self-refrigerating effect approximately 5 F. from its initial temperature of about 70 F. The liquid from the first stage was then passed to the second stage 32 wherein the pressure was again reduced with attendant temperature reduction and vaporization of constituents in relative favorable proportions.

The flashing operation was successively repeated in the third, fourth and fifth stages to a flnal pressure of about lbs. per sqare inch gage and a correspondingly final temperature of about 43 F. By this series of venting operations approximately 93% of the methane and 50% of the Ca constituents in the original liquid charged to the first stage were eliminated. The loss of Ca constituents was about 15% and the loss of C4 constituents was about 5%.

As compared with the use of a fractionating column, the loss of C3 constituents was no greater although a fractionating column would have eliminated substantially all C! constituents without loss of C4 constituents. As in the case of a fractionating column, the vent gases in the instant operation were conducted to a vapor recovery unit such as H of Fig. 1 where the Ca and C4 constituents were recovered.

The sub-cooled liquid from the final stage at 43 F. was used to increase condensation of the separator feed after the water cooling operation described above. The use of this very low temperature liquid for this purpose naturally reduced the loss of C: constituents to the gas from the 500 lb. separator. If a fractionating tower were used for elimination of methane and C: constituents from the heavier constituents, there would have been no material available for this sub-cooling operation. In consequence, the Ca loss from the 500 lb. pressure separator would have been much greater. Since the gas from this separator is recycled to the vapor recovery unit I4 for recovery of Ca constituents, the total C: constituents passing to the unit would be approximately the same in each case; i. e., in the case of the fractionating tower, the separator gas plus tower gas would contain approximately the same quantity of C3 constituents, if not more, than in the case of the vent system where the separator gas and all the vent gases are passed to the vapor recovery unit. Thus, it ca readily be seen that the first cost and operating cost of the multistage vent system is a very small portion of the cost of a fractionating unit which would have to be built for 650 lbs. working pressure and include a 30-plate tower, reboiler, reflux condenser,

accumulator, pumps, and controllers. Thus, it isapparent that through the use of my invention considerable economies in plant investment and operating cost may be effected.

Thus, it will be observed that I have accomplished the objects of my invention. I have provided a method for eliminating, extracting or separating a constituent, constituents or constituent fractions of relatively high volatility from a mixture thereof with constituents of lower volatility through the use of which marked economies in from the spirit of this invention. It is, therefore,

to be understood that this invention is not to be limited to the specific details described.

Having thus described my invention, what I claim is:

Apparatus for obtaining a hydrocarbon liquid of desired volatility from a mixture of gaseous vaporous hydrocarbons comprising in combination; means ior cooling the mixture by indirect eondensingtemperature; means, iorming a series of flash chambers; means for conducting condensate from said cooling means to a first chamber oi said series; means for cooling the condensate in transit to said first chamber; means for. conducting condensate from a preceding to a succeeding chamber of said series; means for supplying absorption medium to each chamber of said series; cooling means in each chamber; means in each chamber of said series providing a tortuous path of flow tor the absorption medium and evaporation products about the cooling means therein; means for conducting nonliquid evaporation product from each said chamber;

heat exchange with a colder fluid medium to a go and, means for conducting condensate from the flnal chamber of said series to said condensate cooling means for use as the cooling medium therein.

KARL FINS'I'ERBUSCH. 

